Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03D—WIND MOTORS
  • F03D7/00—Controlling wind motors 
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F03—MACHINES OR ENGINES FOR LIQUIDS; WIND, SPRING, OR WEIGHT MOTORS; PRODUCING MECHANICAL POWER OR A REACTIVE PROPULSIVE THRUST, NOT OTHERWISE PROVIDED FOR
  • F03D—WIND MOTORS
  • F03D9/00—Adaptations of wind motors for special use; Combinations of wind motors with apparatus driven thereby; Wind motors specially adapted for installation in particular locations
  • F03D9/20—Wind motors characterised by the driven apparatus
  • F03D9/25—Wind motors characterised by the driven apparatus the apparatus being an electrical generator
  • F03D9/255—Wind motors characterised by the driven apparatus the apparatus being an electrical generator connected to electrical distribution networks; Arrangements therefor
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
  • G01R31/40—Testing power supplies
  • G01R31/42—AC power supplies
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P29/00—Arrangements for regulating or controlling electric motors, appropriate for both AC and DC motors
  • H02P29/02—Providing protection against overload without automatic interruption of supply
  • H02P29/024—Detecting a fault condition, e.g. short circuit, locked rotor, open circuit or loss of load
  • H02P29/0243—Detecting a fault condition, e.g. short circuit, locked rotor, open circuit or loss of load the fault being a broken phase
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P9/00—Arrangements for controlling electric generators for the purpose of obtaining a desired output
  • H02P9/006—Means for protecting the generator by using control
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2270/00—Control
  • F05B2270/10—Purpose of the control system
  • F05B2270/107—Purpose of the control system to cope with emergencies
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2270/00—Control
  • F05B2270/10—Purpose of the control system
  • F05B2270/109—Purpose of the control system to prolong engine life
  • F05B2270/1095—Purpose of the control system to prolong engine life by limiting mechanical stresses
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2270/00—Control
  • F05B2270/30—Control parameters, e.g. input parameters
  • F05B2270/327—Rotor or generator speeds
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F05—INDEXING SCHEMES RELATING TO ENGINES OR PUMPS IN VARIOUS SUBCLASSES OF CLASSES F01-F04
  • F05B—INDEXING SCHEME RELATING TO WIND, SPRING, WEIGHT, INERTIA OR LIKE MOTORS, TO MACHINES OR ENGINES FOR LIQUIDS COVERED BY SUBCLASSES F03B, F03D AND F03G
  • F05B2270/00—Control
  • F05B2270/30—Control parameters, e.g. input parameters
  • F05B2270/335—Output power or torque
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
  • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
  • G01R31/34—Testing dynamo-electric machines
  • G01R31/343—Testing dynamo-electric machines in operation
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/70—Wind energy
  • Y02E10/72—Wind turbines with rotation axis in wind direction

Definitions

• a system and method for fault-tolerant control of variable-speed variable-pitch wind turbine is disclosed in which, subsequent to the occurrence of certain common generator faults, the turbine continues producing energy under modified, fault-managed conditions.
• the invention relies on employing torque modulation to locally reduce electrical and/or mechanical stress on fault location in the generator and optimizing wind turbine operating parameters to maintain optimal energy generation under fault conditions.
• the invention results in improved availability of wind turbines.
• MTBF is the mean time between failures, assumed constant, and MTTR is the mean time-to-repair (i.e., duration of the repair).
• MTTR is the mean time-to-repair (i.e., duration of the repair).
• the availability of many wind turbines in use today is around 98% owing to good reliability, maintenance management, and fast repairs. In spite of this, any reduction in the rate of equipment internal failures and any ways to minimize downtime for repairs are of critical importance for owners and operators of wind turbine systems.
• the wind turbine failures can be caused by external factors, such as lightning, fire, and earthquakes, but are most often caused by internal factors, stemming from the turbine equipment.
• Fault tolerance is the ability to continue operation after a fault has occurred.
• a wind turbine equipped with fault-tolerant control system could continue operation, i.e., electricity generation, even after a fault has occurred, and the fault could be repaired at some later, more convenient time without decreasing availability of the generated electricity in the intervening period.
• Fault-tolerance increases MTBF and, hence, the availability of the wind plant.
• This application discloses a system and a method for improving the availability of wind turbines electricity generation by implementing a method for operating a wind turbine generator in the presence of certain commonly encountered generator-fault conditions.
• This application discloses a method for achieving fault-tolerant control of variable-speed variable-pitch wind turbines under the occurrence of several generator faults.
• the generator faults addressed by the method disclosed include but are not limited to those occurring in the stator windings (reduced resistance, or developing short circuit between the stator windings due to aging, temperature cycling, or damage) and rotor defects (either mechanical or electrical defects).
• the listed faults are generally associated with a specific location on the stator or the rotor, such that the peak magnetic flux periodically traverses it during the normal operation of the generator.
• the magnitude of generator torque is reduced either locally around the faulty location on the stator/rotor or globally to a safe value under which the detected fault no longer progresses.
• the reduction of the generator torque includes (a) torque modulation (in time) by means of modulating the effective generator load, and/or (b) reduction of wind power extraction by means of altering the blade pitch.
• Torque modulation means a process in which the magnitude of the rotating magnetic field is modulated as it travels along the circumference of the stator or of the rotor.
• the method and system for fault management of a wind turbine described within is applicable to wind turbines and other energy generation machines. More specifically, the method and system applies to machines using synchronous and induction (asynchronous) generators without departing from the spirit of the invention.
• Variable-speed variable-pitch wind turbines have two operating regions as shown in Figure 13 : A low wind speed region I where as much wind power as possible is captured and high wind speed region ⁇ where the power extracted from the wind is held constant regardless of the wind speed.
• the wind speed at the boundary between regions / and // is referred to as the rated wind speed, or the nominal wind speed V n .
• the wind turbine shuts down (Region III).
• the optimization of energy extraction from the wind is generally accomplished by adjusting the blade pitch angle ⁇ to maximize the aerodynamic performance of the blade rotor and adjusting the resistance to blades rotation as is known in the art.
• Maintaining the wind turbine along the optimal power extraction locus is accomplished by adjusting the generator torque T which resists the spinning of the rotor blades while the blade pitch angle ⁇ is kept constant (set to optimal). This approach is employed in region 7, up to the nominal wind speed V n . Above the nominal wind speed, in region ⁇ , the rotor blade pitch ⁇ is adjusted to limit the angular velocity and the torque to the nominal value and T , respectively.
• Modem wind plants include a number of sensors that continuously monitor the operation of the plant, sense faults, and in recent times, sense also slowly developing failures and pre-failure events.
• early fault warnings include but are not limited to the detection of decreased resistance between adjacent windings on the stator, asymmetry in current draw, or reduced resistance between the stator and chassis, excessive vibration, and eccentricity of the generator rotor.
• fault management protocol mandates that the wind turbine is to be shut down when one of the above-mentioned faults is detected, even if an early warning of a developing problem is issued. Shutting down means rotating the blades at 90° pitch angle and halt the turbine rotation by reducing the aerodynamic torque to zero. Energy production therefore also drops to zero.
• fault location a specific location on the stator or the rotor
• fault location refers to a physical location where the fault has been detected. Fault location is specified by mechanical or electrical angles or angular ranges on the rotor or the stator relative to some predetermined reference.
• the generator fault such as a stator winding fault, is located by specifying a range of mechanical or electrical angles (relative to a reference that is fixed with respect to the stator or to the rotating magnetic field).
• the fault condition at specific location is furthermore characterized with a maximum torque value at the fault location under which the fault is expected not to progress, i.e., expected to remain stable. The torque is proportional to the magnitude of the rotating magnetic flux and to the magnitude of the torque building current.
• the magnitude of the rotating magnetic flux in the stator and/or the magnitude of the torque building current are modulated in position along the circumference of the stator so that they provide a reduced time derivative of flux linkage and possibly a reduced torque at the fault location.
• the location where the reduced torque occurs is fixed relative to the stator.
• the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current is modulated in position along the circumference of the rotor so that they provide a reduced torque at the location of the fault.
• the region of lower torque rotates with the angular velocity equal to the rotor angular velocity.
• the magnitude of the rotating magnetic flux in the stator and/or the magnitude of the torque building current are modulated in position along the circumference of the stator so that they provide a reduced time derivative of flux linkage and possibly a reduced torque at the fault location.
• the location where the reduced torque occurs is fixed relative to the stator.
• the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are set to predetermined values at a fault location along the circumference of the stator or the rotor.
• the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are modulated to predetermined values at positions along the circumference of the stator or the rotor that do not include the fault location.
• the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are reduced to first predetermined values on at least one part of the stator circumference and increased to another predetermined values at positions along the circumference of the stator that do not include said at least one part.
• a generator having a stator operates at a first average torque; after detecting a generator fault condition, the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are reduced to lower predetermined values on at least one part of the circumference of the stator and increased to higher predetermined values at positions along the circumference of the stator that do not include said at least one part, the resulting average torque having a second average value, said first average value being substantially equal to said second average value.
• a wind turbine having at least one blade and a generator having a stator operates at a first average torque value with a first blade pitch; after detecting a generator fault condition, the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are reduced to lower predetermined values on at least one part of circumference of the stator and increased to higher predetermined values at positions along the circumference of the stator that do not include said at least one part, and with a second blade pitch, the torque now having a second average value, said second average torque value being lower than said first average torque value, and said second blade pitch larger than said first blade pitch.
• a generator having a rotor operates at a first average torque value; after detecting a generator fault condition, the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are reduced to lower predetermined values on at least one part of the circumference of the rotor and increased to higher predetermined values at positions along the circumference of the rotor that do not include said at least one part, the resulting average torque having a second average value, said first average torque value being substantially equal to the said second average torque value.
• a wind turbine having at least one blade and a generator having a rotor operates at a first average torque value with a first blade pitch; after detecting a generator fault condition, the magnitude of the rotating magnetic flux and/or the magnitude of the torque building current are reduced to lower predetermined values on at least one part of the circumference of the rotor and increased to higher predetermined values at positions along the circumference of the rotor that do not include said at least one part, and with a second blade pitch, the torque now having a second average value, said second average torque value being lower than said first average torque value, and said second blade pitch larger than said first blade pitch.
• a wind turbine delivers first electrical power, upon an occurrence of a generator fault, the wind turbine is operated in a fault-tolerant mode in which said wind turbine delivers second electrical power, wherein said first electrical power and said second electrical power are substantially equal.
• a wind turbine operates with a first blade pitch and first angular velocity; upon an occurrence of a generator fault, the wind turbine is operated in a fault-tolerant mode characterized with a second blade pitch greater than said first blade pitch, and a second angular velocity lower than said first angular velocity, wherein a torque provided by the generator results in maximum wind energy extraction under which the fault no longer propagates or further develops.
• FIG. 1 shows Stator-controlled squirrel cage induction generator
• - Figure 2 shows Ideal power curve with maximum at power equal P N and power curve due to developed fault with maximum at power equal p ;
• - Figure 3a) shows Power coefficient and Fig. 3b) Torque coefficient for an exemplary 700kW variable-pitch turbine;
• FIG. 4 shows Principal scheme of the control system of a variable-speed variable- pitch wind turbine
• FIG. 5 shows Torque control strategy of a variable-speed wind turbine, where point A denotes the generator cut-in speed;
• FIG. 10 shows Wind turbine output power and pitch angle for healthy and faulty conditions under linear change of the wind speed throughout the wind turbine operating area
• fault occurs at 35 s
• fault occurs at 35 s
• FIG. 13 shows Three distinct control regions of a wind turbine depending on the wind speed
• FIG. 14 shows Wind turbine vs. rotor speed characteristics at different wind speeds.
• FIG. 15 shows Block diagram illustrating the operating sequence when generator fault condition is detected: (a) conventional procedure (prior art), and (b) preferred method disclosed in this application; and
• FIG. 16 shows Block diagram illustrating the preferred method for operating a wind turbine under fault condition.
• Wind turbine in principle consists of a three-blade aerodynamic system mounted on a hub, of a nacelle and of a tower. Inside the nacelle there is a drivetrain with optional gearbox and electrical subsystem with a generator and an electronic converter.
• generators used in wind turbines: doubly-fed induction generator (DFIG) with a wound rotor connected to the grid through slip rings and electronic converter (EC), squirrel-cage induction generator (SCIG) with an EC connected to the stator (see Figure 1) and a direct-drive synchronous generator (SG) coupled to the grid through an EC and with rotor slip-rings for the excitation voltage.
• DFIG doubly-fed induction generator
• SCIG squirrel-cage induction generator
• SG direct-drive synchronous generator
• Fault-tolerant control proposed in this application can be applied to any of listed wind turbine electrical subsystems, assuming that the generator tracks the given torque reference with certain dynamics.
• the generator torque tracking system as a first-order lag system. Every type of generator has its own way of speed control and it can be used to shift operating points in the generator x-axis of speed-torque plane. By using pitch control of a wind turbine aerodynamic torque can be also displaced in the y-axis. This gives the control system the ability to move the operating point with two degrees of freedom in the speed-torque plane.
• the optimum point to which the wind power capture will rise is chosen, which determines the wind turbine rated power (full line in Figure 2).
• operating map of the turbine is parted into a low wind speed region (region I), where all the available wind power is fully captured and high wind speed region (region ⁇ ) where the power output is maintained constant while reducing the aerodynamic torque to the rated value and keeping generator speed at the rated value.
• the ability of a wind turbine to capture wind energy is expressed through a power coefficient C p which is defined as the ratio of extracted power P r to wind power P v :
• the real power coefficient of modern commercial wind turbines reaches values of about 0.48 [11 ].
• Power coefficient data is usually given as a function of the tip-speed-ratio ⁇ and pitch angle ⁇ ( Figure 3). Turbine power and torque are given by [12]
• C Q C P IX , p , R , V and ⁇ are torque coefficient, air density, radius of the wind turbine aerodynamic disk, wind speed and the angular speed of blades, respectively, and
• the task of the control system is to maintain the output power of the wind turbine constant. It can be done by reducing the aerodynamic torque and angular speed of blades by rotating them along their longitudinal axis (pitching). Consequently, power captured from the wind is reduced.
• PI proportional-integral
• PID proportional-integral-derivative
• a fault identification procedure directly outputs the safety torque value to reduce mechanical stress on the damaged rotor bar, together with the location of the fault in the rotor magnetic flux frame (angles ⁇ and ⁇ 2 ).
• a fault identification procedure localizes the damaged part in the stator magnetic flux frame and outputs the maximum allowed machine flux time derivative on the damaged part which can be transferred to the maximum safety torque allowed with respect to the generator speed.
• Torque modulation is shown in Figure 6.
• the torque is reduced to the maximum allowed value defined with a fault condition and, possibly, current generator speed.
• torque is restored to the right selected value T g nonf .
• the value of T g nonf is determined such that the average machine torque is maintained on the optimal level, taking into account the machine constraints.
• Procedure is then periodically executed, with electrical angle period equal ⁇ , since the flux influences the faulty part with its north and south pole peak in each turn. Due to the finite bandwidth of the torque control loop, it is necessary to start reducing the torque prior to reaching the flux angle ⁇ , while the torque is restored to the value T g nonf at the electrical angle ⁇ ⁇ after passing ⁇ 2 .
• Torque transitions between steady-state operating areas are simplified and presented as linear characteristics that correspond to maximum available torque decrease/increase rate determined by maximum possible currents decrease/increase rates, i.e. by the power converter DC link constraints. Considering more realistic torque changes which would be a mix of linear and exponential functions can be also handled but is in the following presentation omitted to facilitate the mathematical treatment of the proposed method.
• Figure 6 also shows that reducing the torque takes less time than restoring it. This is due to the back-electromotive force which opposes the current and thereby aggravates torque restoration.
• Available torque rates for generator torque decrease and increase are denoted with T g _ (a> g ) and T ( ⁇ ) , respectively.
• Equation (10) (or (11)) is not satisfied if the speed co g is large enough (or if there is a relatively large part of the flux circumference under fault influence).
• the torque modulation is shown in Figure 7 and peak torque T * is attained at angle ⁇ * :
• T * and ⁇ * can be expressed as:
• the optimal power point on T m ⁇ g ) which is always on the upper edge of the dashed area, may deviate from this point and thus further improvements in power production outside the point ⁇ co gi > )) ma Y be obtained by using maximum power tracking control along the curve of maximum T m in the speed span [ ⁇ , ⁇ ⁇ ⁇ .
• the interventions in classical wind turbine control that ensure fault-tolerant control are given in Figure 9.
• Algorithms of the slow and the fast fault-tolerant control loops are given in the sequel.
• T g nonf T ⁇ ; if T g nonf > T gn , set
• This section provides simulation results for a 700 kW MATLAB/Simulink variable-speed variable-pitch wind turbine model.
• blade pitching is used in the faulty condition to bring the speed-torque operating point into (fi> ⁇ , ⁇ m (p> ⁇ ⁇ ) ⁇
• FIG. 15 An exemplary method of fault management of a wind turbine and its improvement over prior art is illustrated in Figure 15.
• the wind turbine operates normally, i.e., in normal operation state 301, while continuously assessing the health of the generator, and upon detection 302 of a fault condition in the generator, the wind turbine is shut down to state 303.
• a wind turbine operates in a normal operation state 311, while a generator monitoring subsystem continuously senses the condition of the generator.
• the operating control Upon the detection of a generator fault condition in 315, the operating control initiates fault- management sequence schematically enclosed in the dashed box 316.
• the fault-management sequence comprises of first determining fault-management parameters at 314 based on data provided by the generator monitoring subsystem. If the operation of the turbine is not feasible with the fault-management parameters, the wind turbine is shut down to state 313. If on the other hand, the turbine operation is feasible with the fault-management parameters, the operating control moves to a fault-managed state 312. While in the fault-managed state 312, energy generation continues and the generator monitoring subsystem continues to monitor the state of the generator. Should the detected fault further progress or any new generator fault be detected while in fault-managed state 312, the operating control initiates the fault-management sequence at 314 again.
• Feasible operation means operating the turbine with the generator speed above the generator cut-in speed.
• a method for fault-tolerant control of a wind turbine including a generator comprises: if no generator fault condition is detected, operating said turbine in a normal state; and if a generator fault condition is detected, carrying out the steps of (1) providing fault management parameters corresponding to said detected fault; and (2) determining whether continued operation of said turbine with said fault management parameters is feasible; wherein if said continued operation is determined to not be feasible with said fault management parameters, said turbine is shut down; and if said continued operation is determined to be feasible with said fault management parameters, said turbine is operated in a fault-managed state using said fault management parameters, and progression of said generator fault is monitored, such that if said fault progresses to a predetermined level, new values for said fault management parameters are provided, and step (2) is repeated.
• the algorithm for operation under fault condition comprises of reducing the torque in the faulty part of the generator as it operates and to anticipate such fault-tolerant behavior in overall torque-speed control of the turbine. In this way, the fault development will be evaded or postponed while at the same time it will be possible to optimize the energy production in the non-healthy generator state. This increases wind turbines total energy output and consequently its economic value.
• the preferred method for fault-tolerant control of a wind turbine disclosed in this application comprises of providing a generator including an adaptive control algorithm with two control loops: a fast control loop for modulation of the generator torque along the circumference of the rotating magnetic flux, and a slow control loop that ensures the generator placement at a point of its speed-torque plane where it is possible to: (i) protect the generator and other wind turbine components from further damage by preventing further development of the detected fault and (ii) keep the electrical energy production optimal under emergency circumstances.
• the invention disclosed within applies to all electrical generators used in wind turbines.
• the fault-management control comprises two control loops: the slow control loop and the fast control loop.
• the fast control loop modulates the torque during the rotation of the magnetic flux.
• the slow loop adjusts the generator angular velocity using the average torque and the blade pitch, if necessary in order to protect the generator from further fault development and to optimize the electrical energy production during the fault-managed state.
• the first step is the acquisition of the momentary angular velocity of the generator o g and the associated required generator torque V f 401.
• the fault detection algorithm establishes the location of the fault and the fault severity providing the location in terms of angles ⁇ , ⁇ 2 , and the maximum allowed torque at the fault location (step 402). It is understood that the fault may occur on the stator or the rotor of the generator.
• the generator torque will be modulated to exhibit reduced torque around the location of the fault and an increased torque T nonf in all other areas of the circumference of the magnetic flux.
• control system In the presence of torque modulation, in the preferred method control system first decides what is the higher value of the torque T g nonf (step 403) necessary to maintain unchanged average torque and angular velocity on the generator.
• the higher value T g nonf is capped to the nominal torque value and the torque modulation angles are determined in step 404.
• the numbers in parentheses indicate equations from the previous paragraphs. These equations may vary with the fault type, the one considered here is the stator winding fault of a synchronous generator.).
• the modulation angles are fed to the fast loop 405 to control the torque modulation depending on the position of the rotating magnetic flux, while the same information is also used to compute required angular velocity co gl from the intersection point between T m ⁇ co g ) and normal torque controller characteristics in step 407, the resulting angular velocity ⁇ , is then fed back to the wind turbine control loop, including blade pitch control. If the fault has progressed beyond a predetermined point (not shown in Figure 16) and the turbine operation became unfeasible, the turbine is shut down and the method concludes.
• Feasible operation of the wind-turbine means operation at an angular velocity above the generator cut-in speed.

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